Cyclic adsorption process using centrifugal machines

ABSTRACT

A cyclic adsorption process is provided having pressurization and depressurization steps and driven by one or more centrifugal machines operating under acceleration and deceleration conditions wherein the deceleration rate of the machine is controlled to minimize power consumption and maximize the efficiency of the process. The operating speed of the centrifugal machine during deceleration is matched to the measured ratio pressure conditions so that the centrifugal machine arrives at its minimum operating speed near the point required to begin acceleration.

FIELD OF THE INVENTION

The present invention provides an improved cyclic adsorption system andprocess for separating components of a gas stream using centrifugalmachines. More particularly, the present invention is directed toadsorption processes using centrifugal machines operating under cyclicacceleration and deceleration conditions wherein the deceleration rateof the machine is controlled to minimize power consumption and maximizethe efficiency of the process.

BACKGROUND OF THE INVENTION

Cyclic adsorption processes are well known and are typically used toseparate a more adsorbable component from a less adsorbable component.The typical cyclic adsorption process employs a selective adsorbent toremove at least one component of a gas mixture, employing four basicprocess steps: (1) adsorption, (2) depressurization, (3) purge and, (4)pressurization. The feed fluid, usually a gas, containing the morereadily adsorbable component and a less readily adsorbable component ispassed through at least one adsorbent bed capable of selectivelyadsorbing the more readily adsorbable component at a predetermined(upper) adsorption pressure. The stream exiting the bed at this upperpressure is now concentrated in the less readily adsorbable component,and is removed as product. When the bed becomes saturated with thereadily adsorbable component, the bed is thereafter depressurized to alower desorption pressure for the desorption of the readily adsorbablecomponent, with this component discharged from the system. Suchprocesses are generally used to separate gases such as oxygen ornitrogen from air; hydrocarbons and/or water vapor from feed air gases;hydrogen from carbon monoxide; carbon oxides from other gas mixtures;and the like.

Examples of suitable cyclic adsorption system include, but are notlimited to, pressure swing adsorption (PSA), vacuum swing adsorption(VSA) or vacuum pressure swing adsorption (VPSA) processes which use alow pressure or a vacuum and a purge gas to regenerate the sorbent andtemperature swing adsorption (TSA) processes which uses a thermaldriving force such as a heated purge gas to desorb the impurities.

Traditionally, cyclic adsorption plants, such as VPSA plants, usepositive displacement machines such as rotary lobe type blowersoperating at fixed speeds to move gas through the process. Thesemachines are robust and generally do not experience any significantoperational problems as the pressures and flows change and reverse.However, these machines have low power efficiency and are typically only60-65% efficient.

More recently, applicants and others have proposed the use of moreefficient machines capable of meeting the rigorous requirements of rapidcyclic conditions in place of traditional rotary lobe type machines. Forexample, U.S. Pat. No. 7,785,405B2 discloses systems and processes forgas separation using high-speed permanent magnet variable-speed motorsto accelerate and decelerate centrifugal compressors used in PSA or VPSAprocesses. The centrifugal compressors are driven by direct drivevariable, high speed permanent magnet motors or direct drive variable,high speed induction motors and have efficiencies of approximately 85%.Such compressors can accelerate from low-speed to high-speed anddecelerate from high-speed to low-speed at very rapid rates offering amagnitude improvement over the capabilities of conventional machineswith conventional motor/gear box systems.

One challenge in using centrifugal compressors is that the compressorperformance is very sensitive to rapid changes in pressure, such as thepressure changes that typically occur during cyclic adsorptionprocesses. When the process cycle requires the compressor speed todecelerate due to falling pressure ratios, the control system orcontroller typically direct the variable frequency drive (VFD) todisable energy input to the motor allowing the drive train (motor rotorand compressor impeller) to “free-wheel” decelerate (coast) down to itsminimum speed without consuming power. If the drive train reaches theminimum speed too quickly, such as before the completion of the fallingpressurization equalization step, the VFD re-enables energy input to themotor thereby consuming unneeded power. Power as used herein refers toelectrical power.

It has now been found that by properly operating the machine duringdeceleration one can avoid reaching the design minimum operating speedtoo quickly. By avoiding reaching minimum operating speed before thepressure ratio across the machine starts rising, the machine and can beoperated at peak efficiencies. Thus, one objective of this invention isto match the deceleration rate of the compressor/drive train to thedecreasing pressures ratio across the machine so that the centrifugalmachine arrives at its minimum operating speed near the point requiredto begin acceleration/reacceleration and, preferably, at the pointrequired to accelerate/reaccelerate and along the compressor's bestefficiency line (as shown in FIG. 5). This eliminates the unnecessarypower consumption used during machine idling time which occurs afterdeceleration as further described below.

One method to match the compressor deceleration speeds to the decreasingpressures ratio is through the use of sophisticated control systems thatcontinuously measure, or monitor at frequent points, the machine speedand the pressure ratio. The rotation of the drive train of thecentrifugal machine is then controlled using dynamic braking (energy isfed to a braking resistor) or regenerative braking (energy is fed backinto the power grid or stored in a flywheel until needed) based uponinstructions received from the control system to reduce the decelerationrate and arrive at its minimum operating speed near or at the pointrequired to begin acceleration/reacceleration.

However, the preferred method to control the rotation of the drive trainis through the use of aerodynamic braking which can be controlled byeither increasing or decreasing the feed fluid mass flow to thecompressor which in turn increases or decreases the amount of work doneto the fluid by the compressor impeller. This is accomplished by the useof flow control valves, preferably at the suction inlet of thecompressor, and matching the mass flow to the desired deceleration ratethrough the operation and movement of the valves. The valves are openedto release process fluid and thereby the mass flow or closed to reducemass flow. In this way, the aerodynamic braking that occurs duringfree-wheel deceleration can be reduced by suction throttling one or morevalves at fixed position. One or more control valves can be employeddepending on the system and number of adsorber vessels employed. Thisapproach is economical and simple to commercially implement.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an improved process, and system for theuse of centrifugal machines in cyclic adsorption processes. Thisinvention provides a method for realizing additional power benefitsassociated with using centrifugal machines in place of conventionalpositive displacement machines by minimizing the unnecessary use ofpower during deceleration.

According to one embodiment of this invention, a cyclic adsorptionprocess is provided comprising:

-   -   at least one adsorber vessel containing at least one adsorber        bed undergoing the steps of pressurization and depressurization        and driven by at least one centrifugal machine;    -   the centrifugal machine undergoing the steps of acceleration and        deceleration in accordance with the requirements of the cyclic        adsorption process, and    -   a controller for receiving data signals for the process        conditions and sending instructions to the centrifugal machine        in response to the conditions; the improvement comprising        measuring the pressure ratio conditions during deceleration and        matching the operating speed of the centrifugal machine to the        measured pressure ratio conditions so that the centrifugal        machine arrives at its minimum operating speed near the point        required to begin acceleration.

In another embodiment, a cyclic adsorption process comprising (i)pressurizing a feed gas and passing the feed gas through an adsorbentbed which adsorbs at least one readily adsorbed gas while passingthrough at least one less adsorbed gas as a product gas and (ii)depressurizing the adsorbent bed containing the at least one morereadily adsorbed gas to desorb the adsorbed gas from the adsorbent bed;wherein the process comprises

-   -   passing the feed gas or the product gas through at least one        centrifugal machine for the pressurization and depressurization        steps, the at least one centrifugal machine undergoing        acceleration and deceleration conditions,    -   measuring the pressure ratio conditions during the deceleration        conditions and sending a signal to a controller for receiving        data signals with the pressure ratio conditions data,    -   having the controller convert the data signals to instructions        using a predetermined algorithm designed to instruct the        centrifugal machine to arrive at the minimum operating speed at        the point required to accelerate along the best efficiency line,        and    -   sending the instructions from the controller to at least one        flow control valve which moves in response to the instructions        to move to adjust the mass flow of the process gas to vary the        operating speed of the centrifugal machine so as to obtain its        lowest operating speed at the point required to begin        acceleration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating how the prior art centrifugal machineoperates during the period through deceleration to acceleration duringthe steps of depressurization and pressurization in a typical VPSAoxygen process.

FIG. 2 is a graph showing a centrifugal compressor performance map withcurves indicating the flow generated by the centrifugal machine forvarious operating speeds and pressure ratios across the machine and thedeceleration line during operation of the process shown in FIG. 1.

FIG. 3 is a schematic of a two-vessel vacuum pressure swing adsorptionsystem and process according to one embodiment of the present invention.

FIG. 4 is a graph illustrating how the centrifugal machine operatesduring the period through deceleration to acceleration during the stepsof depressurization and pressurization in a preferred embodiment of thepresent invention.

FIG. 5 is a graph showing a centrifugal compressor performance map withcurves indicating the flow generated by the centrifugal machine forvarious operating speeds and pressure ratios across the machine and thedeceleration line during the operation of the process shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the present invention provides an improved systemand process for the use of centrifugal compressors in cyclic adsorptionprocesses. This system and process uses at least one centrifugalcompressor optimized to the operating requirements of the process cycle.More specifically, the centrifugal compressor is generally synchronizedwith the power requirements during the deceleration and acceleration (orre-acceleration) cycle steps such that the power is disengaged duringdeceleration and only reengaged upon acceleration. This minimizes powerconsumption and maximizes process efficiency in rapid cyclic adsorptionprocesses.

Deceleration generally occurs when rapid changes in vessel pressure,such as during falling pressure equalization, result in decreasingpressure ratios across the centrifugal machine. For example, in singlebed oxygen production systems, deceleration will occur in stepsincluding oxygen recovery depressurization, oxygen recovery overlapevacuation, evacuation, oxygen recovery overlap feed, and productpressurization overlap feed. In multiple bed oxygen production systems,deceleration will occur in steps including falling pressureequalizations, falling pressure evacuations with overlap equalization,falling pressure evacuations, raising pressure evacuations with overlapequalization, rising pressure feedings with overlap equalization, andrising pressure feedings with overlap product pressurization.

The cyclic adsorption processes useful herein will have at least onevessel containing at least one adsorbent bed therein (herein an“adsorber vessel” and an “adsorber bed” respectively) and the adsorberbed can have one or multiple layers and types of adsorbents. Theprocesses can include the separation of fluids for a wide variety ofapplications such as used to separate contaminates from end products inthe air separation, refining, natural gas, chemical and petrochemicalindustries. Typically, the processes are gas separation processes andexamples of preferred processes include PSA, VSA, and VPSA processes.

Adsorbents suitable for such processes are generally well known andinclude molecular sieves, aluminas, silicas, zeolites, catalysts with orwithout associated (coated or impregnated) active metals or metaloxides, and the like. For PSA, VSA, and VPSA air separation processes,suitable adsorbents include, but are not limited to, A, X, and Y typezeolites and various ion exchanged forms of these zeolites, as well assilica-alumina, alumina, silica, titanium silicates, phosphates andmixtures thereof.

For illustrative purposes and without limiting the scope of thisinvention, a typical VPSA process for separating oxygen from air isdescribed herein. The VPSA process is one wherein an adsorber bedundergoes the following steps:

-   -   1) The adsorber bed is pressurized to a desired pressure wherein        nitrogen is readily adsorbed by the adsorbent as the feed air is        passed across the bed;    -   2) Product gas rich in oxygen is produced as the nitrogen in the        feed air is adsorbed;    -   3) The bed containing the adsorbent is evacuated to a low        pressure (typically under vacuum) wherein the adsorbed nitrogen        is desorbed from the adsorbent in the adsorber bed; and,        preferably,    -   4) A purge gas is passed through the bed to remove any remaining        nitrogen.        The cycle time is understood by the skilled person to mean the        amount of time needed to complete one cycle; e.g., the process        steps in order and then return to the starting condition.

Some adsorption processes will have more steps or multiple adsorber bedsand often use one or more compressors/blowers for each of thepressurization and depressurization steps. If the VPSA plant containstwo or more adsorber vessels, each vessel undergoes the above steps;however, the vessels are operated out of phase so that while one vesselis producing product the other is being regenerated. Also, in a twoadsorber vessel process, two compressors are typically used wherein onecompressor is dedicated to feeding gas to the first adsorber vesselwhile the other is dedicated to evacuating the second adsorber vessel;often referred to as a feed compressor and a vacuum compressor,respectively.

Regardless of whether a single vessel, two vessels, or multiple vesselsare used, the pressures and flows within the process change quickly asthe process cycles from adsorption to desorption. Generally, thepressure of a vessel will change from a low pressure condition of at orbelow atmospheric, such as about 6 to 8 psia, to a high pressurecondition of above atmospheric, such as about 19 to 24 psia, in a rapidperiodic cycle time, such as in less than one minute. Some adsorptionprocesses can require even wider spans of pressures and/or vacuums insimilar rapid cycle times.

Cyclic adsorption systems have at least one adsorber bed that iscyclically pressurized by a positive displacement feed compressor andsometimes evacuated by a separate vacuum compressor. In the presentinvention, one centrifugal machine, designed for variable-speed, directdrive operation, is used for both pressurization and depressurization ofa single adsorber vessel and multiple vessel systems will have separatecentrifugal machines; one for feeding gas to the vessels and one forevacuating the vessels.

Centrifugal compressors, sometimes referred to as radial compressors,are well known. They operate at high speeds and generate high pressurerises. The term “centrifugal machines” is used herein to describe theoperating machine which includes the compressor with impeller and themotor/drive system. The term “compressor” is used herein to refer to thecompressor, impeller, shroud and volute. The centrifugal machines havecentrifugal compressors driven by direct drive variable, high speedpermanent magnet motors having VFDs which permit the compressor tocyclically accelerate from a low operating speed to a high operatingspeed and decelerate from a high operating speed to a low operatingspeed at rapid rates as required by current PSA, VSA, or VPSA cycletimes. The term “centrifugal machines” is also intended to includecentrifugal compressors driven by variable, high speed induction motorsusing direct drive systems. Both of these motors are capable ofoperating the compressors at high speeds such as greater than 5000 RPM,preferably greater than 10,000 RPM, and most preferably greater than15,000 RPM. Preferably, for VPSA, VSA, and PSA processes, the lowoperating speed will be not more than 7000 RPM and the high operatingspeed will be greater than at least 13,000 RPM.

The centrifugal compressors used here can have single or multiplestages, can have various impellers or blade configurations, and can beconfigured to operate in association with one or more beds. Thecentrifugal machines can also be used in combination with other positivedisplacement machines. However, one centrifugal machine is typicallyused per service and it is preferred that the centrifugal machine isused in the absence of a positive displacement machine.

FIG. 1 is a graph illustrating how the prior art centrifugal machineoperates during the period through one half cycle in a typical VPSAoxygen process. The compressor drive train speed and power consumptionare plotted as a function of time expressed as a fraction of the totalcycle time. Referring to FIG. 1, when the process cycle requires thecentrifugal machine to decelerate due to falling pressure ratiorequirements, the VFD disables energy input to the machine, allowing thedrive train to free-wheel decelerate down to its designed minimum speedwithout consuming any electrical power as illustrated between points Cand A in FIG. 1. But if the drive train reaches the minimum speed tooquickly (e.g., before the pressure ratio across the machine starts torise), the VFD will re-enable energy input to the motor and hold thedrive train at the minimum speed line as shown at points A through B inFIG. 1 consuming power unnecessarily. This time, shown as the periodbetween points A and B, is referred to herein as the “idle time” andshould be minimized. Idle time is defined here as the period of timeafter deceleration wherein power is provided to the centrifugal machinefor the purpose of maintaining compressor speed at a value above theminimum speed required by the process (typically 30% to 40% of thedesign speed). Electrical power provided to the motor during the idletime is wasted since power is consumed without useful work being done.In order to achieve improved efficiency in a typical cyclic adsorptionprocess, the idle time is preferably less than eight (8) percent of thetotal cycle time and, for PSA, VSA, or VPSA processes, preferably lessthan 3 seconds, more preferably less than 1 second.

An alternate way of illustrating the problem is shown in FIG. 2 whereinthe cycle of the compressor is shown in a plot of % pressure ratioplotted against a non-dimensional mass flow rate. The line connectingpoints B and C is the best efficiency line. The flow coefficient is 1.0at a speed that is 100% of the design speed and design pressure ratio.From points C to A, the centrifugal machine is decelerating. As thepressure ratio falls, the flow coefficient may actually first increasebefore decreasing as shown. From point A to point B, the machineoperates along its minimum constant speed line (here, 40% of designspeed shown as 0.4). As the pressure ratio increases, the flowcoefficient decreases as shown. From points B to C, the machine isaccelerating. As the pressure ratio increases, the flow coefficient alsoincreases.

FIG. 3 is a schematic of a two-bed VPSA system according to oneembodiment of the present invention for the production of oxygen fromair. Since the basic process is well known, only key elements of theprocess will be described. According to FIG. 3, a VPSA system 20includes feed compressor 22, adsorber bed 40, and vacuum compressor 50to efficiently produce the less selectively adsorbed gas, oxygen. Atleast one of the feed compressor 22 and vacuum compressor 50 is acentrifugal machine, preferably vacuum compressor 50. In a morepreferred embodiment, both feed compressor 22 and vacuum compressor 50are centrifugal machines although those skilled in the art will alsounderstand that for PSA and other non-vacuum systems, vacuum compressor50 would not be utilized. Compressors 22 and 50 are driven by permanentmagnetic motor 29 with VFD 33 and permanent magnetic motor 51 with VFD53, respectively, as shown.

Referring again to FIG. 3, feed centrifugal compressor 22 includes asingle-stage compressor driven by permanent magnet motor 29, havinginlet 24 for drawing air feed gas directing a pressurized airflowthrough a feed air aftercooler 27, and then through discharge manifold26 to respective parallel inlet lines 28 and 30. Respective first andsecond pressurizing valves 32 and 34 are plumbed in the distal ends ofthe respective lines to selectively pressurize respective portions ofthe adsorber system 40. A vent valve 36 connects to an intermediateportion of manifold 26 to selectively bypass airflow away from theadsorber system 40. The valves are sequenced through a programmablelogic controller 31 according to timing corresponding to the processsteps for the method of the present invention.

Adsorbent system 40 comprises a dual vessel system, with adsorbervessels A and B each containing at least one adsorber bed (not shown)and having respective bottom portions 42 and 44 disposed downstream ofthe respective first and second pressurizing valves 32 and 34 in analternating parallel arrangement. Respective top portions 43 and 45provide a convenient interface for connecting a product supply mechanism60 comprising a single product surge tank 66. As mentioned above,alternative systems in accordance with the present invention couldemploy a single adsorber vessel or multiple vessels.

At least one adsorber bed including one or more adsorbent layers ormaterials is contained in each adsorber vessel (A and B), preferably ofthe radial flow type. Radial flow vessels are known and include anenlarged feed end of overall asymmetric cross-section of the gas flow.Radial flow vessels accommodate large gas flow ranges and provide only alow pressure drop across the bed in the direction of gas flow. Suchvessels also provide a more uniform flow distribution through the bedand typically offer a restrained bed with an enlarged inlet area. Itshould be noted, however, that alternative flow vessels such as axial orhorizontal beds can be used in the present invention.

The vacuum compressor 50 is plumbed to respective first and seconddepressurizing valves 52 and 54 that connect to vacuum manifold 56. Thevalves are plumbed in parallel opposing relationship to the first andsecond pressurizing valves 32 and 34. Like the pressurizing valves, thedepressurizing valves are sequenced by controller 31. The manifoldterminates in a single stage vacuum compressor 50 is preferably acentrifugal vacuum compressor, directly driven by a high-speed permanentmagnet motor 51 designed for variable-speed operation, and evacuatesrespective beds A and B during the predetermined cycle steps. Theoperation of this system is more fully shown and described in U.S. Pat.No. 7,785,405.

As can be appreciated from FIG. 3, P₁ can remain constant (e.g., atambient conditions) while P₂ will be responsive to conditions inadsorber vessels A and B (P₂ can vary or remain constant duringpressurization, depressurization and during product make steps). As P₂varies, the ratio of P₂/P₁ will likewise vary. Similarly, P₄ can remainconstant (e.g., at ambient conditions) while P₃ will be responsive toconditions in the adsorbent vessels (P₃ can vary or remain duringpressurization, depressurization and during product make steps). As P₃varies, the ratio of P₄/P₃ can likewise vary. Thus, the pressure ratiosfor the feed and vacuum compressors can vary or remain constant based onthe conditions in vessels A and/or B. Feedback to controller 31regarding the pressure ratios can allow for the compressor operatingspeed to be adjusted appropriately. Accordingly, by continuously varyingthe compressor speeds to match the pressure ratio requirement whichitself is varying because of the steps of depressurizating/evacuatingand pressurizing the adsorber vessels A and B, the compressors can beoperated near, and preferably at, their peak efficiencies from 100%design speed to a substantially lower speed. This is easily accomplishedby the skilled person using the information, calculations andperformance maps which are stored in (e.g., hard-coded) the controller31, which then sends a signal to the VFD and associated drive train. Itwill be appreciated that in the exemplary VPSA system for oxygenproduction shown in FIG. 3, P₄ and P₁ could be at or near ambientconditions.

With continued reference to FIG. 3, the product supply mechanism 60includes respective first and second product outlet valves 62 and 64disposed at the top of the respective top portions 43 and 45 of vesselsA and B to direct the oxygen product flow from each bed to purge theother bed, equalize the pressure in the other bed, or flow to surge tank66 for storage. Isolation valve 68 interposed between surge tank 66 andoutlet valves 62 and 64 cooperates with outlet valves 62 and 64according to sequencing commands from the controller 31 to effect theproper purge and/or equalization procedures.

In order to minimize the use of unneeded power by the centrifugalmachine(s) during deceleration, flow control or throttle valves 55, 52and 54 are placed at the suction inlet to the centrifugal machines.Valve 55 is a new valve for purposes of this invention while valves 52and 54 are replacement valves for the on/off pressurizing valvespreviously used. Valves 55, 52 and 54 are preferably placed in thesuction piping at a distance equivalent to the diameter of two or moretimes the suction pipe diameter and placed upstream of the compressor tominimize turbulence as the fluid (air) enters the compressor.

When the adsorption cycle requires the compressor speed to deceleratedue to falling pressure ratio requirements, controller 31 directs VFD 33to disable energy input to the machine, allowing the drive train tofree-wheel decelerate to its minimum speed without consuming anyelectrical power as illustrated between points C and A in FIG. 1. But ifthe deceleration rate is too fast and the drive train reaches theminimum speed too quickly (e.g., before the pressure ratio across themachine starts to rise), the controller 31 is programmed (based on theminimum design speed) to direct VFD 33 to re-enable energy input to themotor and hold the drive train at the minimum speed line as shown atpoints A through B in FIG. 1 consuming power unnecessarily. An alternateway of illustrating the problem is shown in FIG. 2 wherein the cycle ofthe compressor is shown in a plot of % pressure ratio plotted against anon-dimensional mass flow rate. The idle time is shown as the timeperiod between points A through B and the power consumption associatedwith this idle time should be reduced or preferably eliminated.

Referring again to FIG. 3, flow control values 55, 52 and 54 areadjusted to the extent and degree necessary to reduce the inlet density,and thus adjust the process gas mass flow to the centrifugal machines.The control valves can be operated to open or close, but that more oftenthe valves are closed to reduce the inlet density and reduce mass flowas described in this embodiment. By reducing the mass flow of gas to thecentrifugal compressors 22 and 50, the amount of work done to the gas bythe impeller is reduced (i.e., less aerodynamic braking) thereby slowingthe deceleration rate of the compressor(s) drive train as illustrated inFIG. 4. The deceleration from points C to A is slowed by closing thecontrol valves such that point A equals point B, which is the point ofacceleration from points B to C. Thus, flow control valves 55, 52 and 54are closed to the extent and degree necessary to reduce the mass flow ofthe feed gas as needed to reduce the deceleration rate of the impellerand the centrifugal machine. The deceleration rate is preferably matchedto the pressures ratio(s) so that the centrifugal machine does not fullydecelerate before the pressure ratio rises and thereby arrives at itsminimum operating speed near, or preferably at, the point required tobegin acceleration/reacceleration. Further, the centrifugal machinepreferably arrives at the minimum speed along its best efficiency line.

In a typical operation in one embodiment of this invention, the pressureratio conditions are measured during the deceleration step with apressure measuring device which sends a data signal to the controller.The controller converts the data signals into instructions using apredetermined algorithm designed to instruct the centrifugal machine toarrive at the minimum operating speed at the point required toaccelerate along its predetermined best efficiency line. The controllersends the instructions to at least one flow control valve which moves inresponse to the instructions adjusting the mass flow of process gas tovary the operating speed and reduce the deceleration rate of thecentrifugal machine to obtain its lowest operating speed near or at thepoint required to begin acceleration.

As used herein, “near the point required to begin acceleration” meansthat the minimum pressure ratio achieved across the machine occurs atsubstantially the same point in time as the machine reaches its minimumoperating speed, such as within a three seconds, and “at the pointrequired to begin acceleration” means that the minimum pressure ratioachieved across the machine occurs at the same point in time, within 1second, as the machine reaches its minimum operating speed. These pointsare determined from the compressor performance map as described below.

The required movement of the suction control valves necessary to releasefluid/gas to slow down the deceleration rate of the drive train andenable the compressor to arrive at the minimum operating speed at thepoint required to accelerate along the best efficiency line can beexperimentally determined using the information plotted on FIG. 4 andFIG. 5 and automatically controlled using a feedback control algorithmin the controller. The pressure ratio conditions are compared to thepredetermined best efficiency operation for the given centrifugalmachine and adjusted as appropriate. The closer the final decelerationpoint (minimum operation speed) is in time to the point required tobegin acceleration as shown in FIG. 4, the less power is wasted byeliminating the power consumption during the idle time. It is preferredthat the final deceleration point A be substantially the same as thebeginning acceleration (or re-acceleration) point B and more preferredthat the final deceleration point is the same as the beginningacceleration point thereby eliminating compressor idle time.

It is understood that one objective of this invention is to avoid idletime and match the final deceleration point to the point at which themachine reaccelerates. In other words, these two points should occursimultaneously. However, it is difficult to achieve such precision inpractice with current technology and one skilled in the art understandsthat small variances are to be expected. Overall, it has been observedthat the operating speed of the centrifugal machine during decelerationis maintained at a value above the minimum speed permitted by therequirements of the process for a time of less than eight (8) percent ofthe total cycle time.

It should also be apparent to those skilled in the art that the subjectinvention is not limited by the two-bed VPSA process for the separationof oxygen from air as described herein which has been provided to merelydemonstrate the operability of the present invention. The presentinvention can be employed for any cyclic adsorption process using one ormore centrifugal machines and one or more adsorber vessels and theselection of suitable adsorption processes, process conditions, processcycles, cycle times, vessel size, and the like can be determined by oneskilled in the art from the specification without departing from thespirit of the invention as herein described. The scope of this inventionincludes equivalent embodiments, modifications, and variations that fallwithin the scope of the attached claims.

What is claimed is:
 1. In a cyclic adsorption process comprising: atleast one adsorber vessel containing at least one adsorber bedundergoing the steps of pressurization and depressurization and drivenby at least one centrifugal machine, the centrifugal machine undergoingthe steps of acceleration and deceleration in accordance with therequirements of the cyclic adsorption process, and a controller forreceiving data signals for the process conditions and sendinginstructions to the centrifugal machine in response to the conditions;the improvement comprising measuring the pressure ratio conditionsduring deceleration and matching the deceleration rate of thecentrifugal machine to the decreasing pressure ratio conditions duringdeceleration so that the centrifugal machine arrives at its minimumoperating speed near the point required to begin acceleration.
 2. Theprocess of claim 1 wherein the centrifugal machine accelerates along itsbest efficiency line.
 3. The process of claim 1 wherein the operatingspeed of the centrifugal machine during deceleration is maintained at avalue above the minimum speed permitted by the requirements of theprocess for a time of less than eight (8) percent of the total cycletime.
 4. The process of claim 3 wherein the process is a PSA, VSA, orVPSA process and the time of less than, eight (8) percent of the totalcycle time is less than 3 seconds.
 5. The process of claim 1 wherein thedeceleration rate of the centrifugal machine is matched with thedecreasing pressure ratio conditions by the controller; receiving datasignals indicating the pressure ratio conditions during deceleration,comparing the decreasing pressure ratio conditions to the decelerationrate of the centrifugal machine, and communicating to the centrifugalmachine in response to the decreasing pressure ratio conditions,instructions to a control system which matches the deceleration rate tothe decreasing pressure ratio conditions so that the centrifugal machinereaches the final deceleration point at the same time as the pointrequired to begin acceleration.
 6. The process of claim 5 wherein thecontrol system is one or more flow control valves which can be operatedto control the mass flow of the process gas as required to reduce thedeceleration rate of the centrifugal machine.
 7. The process of claim 6wherein the flow control valves are operated by the controller whichreceives data signals indicating pressure ratio conditions in the systemand communicates instructions to the valves in response to the pressureratio conditions thereby controlling the deceleration rate to reach thefinal deceleration point near the point required to begin acceleration.8. The process of claim 7 wherein the deceleration rate is matched tothe pressures ratio conditions so that the idle time is less than eight(8) percent of the total cycle time.
 9. The process of claim 1 whereinthe process is a VPSA, VSA, or PSA process.
 10. The process of claim 9wherein the process is a VPSA process for the separation of oxygen fromair.
 11. A cyclic adsorption process comprising (i) pressurizing a feedgas and passing the feed gas through an adsorbent bed which adsorbs atleast one readily adsorbed gas while passing through at least one lessadsorbed gas as a product gas and (ii) depressurizing the adsorbent bedcontaining the at least one more readily adsorbed gas to desorb theadsorbed gas from the adsorbent bed; wherein the process furthercomprises: passing the feed gas or the product gas through at least onecentrifugal machine for the pressurization and depressurization steps,the at least one centrifugal machine undergoing acceleration anddeceleration conditions, measuring the pressure ratio conditions duringthe deceleration conditions and sending a signal to a controller forreceiving data signals with the pressure ratio conditions data, havingthe controller convert the data signals to instructions using apredetermined algorithm designed to instruct the centrifugal machine toarrive at the minimum operating speed at the point required to beginacceleration along the best efficiency line, and sending theinstructions from the controller to at least one flow control valvewhich moves in response to the instructions to move to adjust the massflow of the adsorbed gas to vary the rate of deceleration of thecentrifugal machine so as to obtain its lowest operating speed at thepoint required to begin acceleration.
 12. The process of claim 11wherein the flow control valve closes to reduce the mass flow of theprocess gas to slow the rate of deceleration of the centrifugal machine.13. The process of claim 11 wherein the rate of deceleration of thecentrifugal machine is reduced to obtain the centrifugal machine'slowest operating speed within 1 seconds of obtaining the minimumpressure ratio across the machine.
 14. The process of claim 11 whereinthe centrifugal machine obtains its lowest operating speed atsubstantially the same point in time as the minimum pressure ratio isachieved.
 15. The process of claim 11 wherein the cyclic adsorptionprocess is the production of oxygen from air.
 16. The process of claim14 wherein the process is a VPSA process.
 17. In a cyclic adsorptionprocess for separating air comprising: at least one adsorber vesselcontaining at least one adsorber bed undergoing the steps ofpressurization and depressurization and driven by at least onecentrifugal machine, the centrifugal machine undergoing the steps ofacceleration and deceleration in accordance with the requirements of thecyclic adsorption process, and a controller for receiving data signalsfor the process conditions and sending instructions in response to theconditions; and wherein the adsorber bed is pressurized to a desiredpressure and nitrogen is readily adsorbed by the adsorbent as the feedair is passed across the bed; oxygen is produced as the nitrogen in thefeed air is adsorbed; and nitrogen is desorbed from the adsorbent in theadsorber bed; the improvement comprising using a control system tocontrol the deceleration rate of the centrifugal machine in response todecreasing pressure ratio process conditions occurring duringdeceleration so that the centrifugal machine arrives at its minimumoperating speed near the point required to begin acceleration.
 18. Theprocess of claim 17 wherein the data signals about the processconditions include pressure ratio conditions measured duringdeceleration by a pressure measuring device.
 19. The process of claim 17wherein the control system is selected from the group consisting of adynamic braking system, a regenerative braking system and an aerodynamicbraking system.
 20. The process of claim 19 wherein the control systemis an aerodynamic braking system.
 21. The process of claim 20 whereinthe control system comprises one or more flow control valves operated onresponse to instructions from the controller to control the mass flow ofthe process gases.
 22. The process of claim 20 wherein the one or moreflow control valves increase or decrease the feed gas mass flow to thecentrifugal machine upon instructions received from the controller. 23.The process of claim 17 wherein the power to the centrifugal machine isdisengaged during deceleration and reengaged at the point required tobegin acceleration along its best efficiency line.